fix(x402): preserve handled failures after waiter cancellation

[nkavian]

Summary

Fix Python 3.14 exception diagnostics after a caller cancels an x402 capability refresh or a prepared-payment wait. The shared operation can still fail after cancellation; that error remains handled by the SDK and available to other callers.

Use the same shared-task waiting pattern as TAP verification, without changing payment requests, approval cancellation, cache lifetimes, or hook execution. Leave bounded approval cleanup unchanged.

Verification

• Add eight regression cases covering Seller configuration/capabilities, Buyer capabilities, and application hooks, with and without another waiting caller.
• Verify preserved error identity, cached hook errors, one shared request, and no implicit approval cancellation.
• Run the full Python 3.11–3.14 repository checks, package-consumer checks, and shared conformance suites.

No public API, dependency, version, or release changes.